WASHINGTON (AFP) - The United Nations' top expert on worldwide human trafficking called Tuesday for the negotiated release of Nigerian schoolgirls held by Boko Haram rebels, amid worries they might be sold off.

"The elements of trafficking are there," said Joy Ngozi Ezeilo, the UN Human Rights Council's special rapporteur on the issue, in a conference call with US-based foreign journalists.

"We cannot do politics with the lives of these young girls," said Ms Ezeilo, a Nigerian herself, adding that she favours a negotiated solution rather than a military assault "that might jeapordise the lives of these girls."

"What should be paramount in our minds is how to bring them safely back home," she said. "They are minors. They have to be at home (and) they deserve to be in school... their education should not be in any way compromised.

Boko Haram staged the mass abduction on April 14 in the north-eastern Nigerian town of Chibok. In a video obtained May 5 by AFP, its leader Abubakar Shekau threatened to sell off the more than 200 young captives as "slaves."

In another video, seen last weekend, Shekau said the girls would be held in return for the release of detained Boko Haram rebels.

Ms Bisa Williams, deputy assistant secretary of state in the US State Department's African affairs bureau, participating in the same conference call, said a "nexus" exists between human trafficking and terrorism.

But in the case of the missing Nigerian school girls, she said, "it's far too early to state that this is a trafficking incident" - which for the United States would entail slavery, sexual exploitation or forced labour.

"We don't know where these girls are," Williams said.